High-voltage winding



Oct. 12, 1937. .1. F. CALVERT HIGH VOLTAGE WINDING Filed Nov. 15, 1935 3 Sheets-Sheet l INVENTOR J07? F. Jaime/"2.

ATTORNEY 0a. 12, 1937. J. F. CALVERT 2,095,790

HIGH VOLTAGE WINDING Filed Nov. 13, 1935 3 Sheets-Sheet 2 WITNESSES:

INVENTOR Job/2 FT Calvert.

WNW

ATTORNEY Oct. 12, 1937. .1. F. CALVERT H IGH VOLTAGE WINDING 3 Sheets-Sheet 3 Filed Nov. 13, 1955 INVENTOR Ja/Erz 1700/0677.

WITNESSES: f

ATTORNEY Patented Oct. 12, 1937 UNITED STATES PATENT OFFICE HIGH-VOLTAGE WINDING sylvania Application November 13, 1935, Serial No. 49,519

16 Claims.

My invention relates to windings for dynamoelectric machines, and it has particular relation to high-voltage windings for polyphase machines.

In the United States, the larger polyphase generators have been built for potentials of 16,500 to 24,000 volts, which have been found to be the most economical voltage from the standpoint of the insulation and construction of the generator. Generators wound for voltages appreciably higher than the present standard are larger in diameter, longer in length, and more costly than units wound for the most economical voltage. However, the difficulties associated with switch- 7 ing equipment, cables, and station structures, in 1 5 handling alternating currents of large magnitudes at the generating stations, emphasize the need for large-capacity generators wound for higher voltages of from 27,600 to 33,000 volts, or even higher. I Long experience with generators insulated with mica insulation in the slots has thoroughly demonstrated that there is an ample factor of safety on the slot insulation. Means have also been developed for bracing and insulating the endturns or end-connections so that the insulation at these points will also safely withstand the potential stresses imposed upon it.

In high-voltage machines, however, there is a more serious difiiculty in connection with corona,

which must be kept small at the normal operating voltage because of the resulting generation of nitrous oxide, in an air-cooled machine, which is particularly disadvantageous in machines having a closed-circuit system of ventilation, which is a preferred form of ventilation for these ma chines. In such machines the nitrous oxide is retained in the enclosure, in the presence of an excessive amount of moisture, gradually increasing in concentration, and forming weak acids which attack the fins of the air-cooler and which deteriorate the organic insulation on the endwinding connections, phase-connecting rings or group-connectors, and bracing members.

Not only must the corona be kept smallat the normal operating voltage, but it is very desirable also to avoid it at the prescribed test voltage of twice normal voltage plus 1000 volts, particularly in machines designed for very high voltages, because corona becomes particularly vicious in its nature at extremely high voltages, developing a tendency to bunch badly and to concentrate at one point, so that considerable damage can be done, at least to the outward appearance of the insulation, if corona is present during the minute w for which the test voltage is applied.

Means have heretofore been developed for preventing the occurrence of corona on the straight parts of the coil-sides, where the coil-insulation comes into contact with the iron of the stator slots. At one time, this was the principal source of corona; but it has been effectively eliminated by the application of a coating of semi-conducting material over the insulation of the coil-sides, as set forth in two Patents Nos. 1,784,989 and 1,784,990, issued to C. F. Hill on December 16, 1930.

If this external coating of semi-conducting material were extended over the end-connections, corona problems could be eliminated there also, as well as on the straight coil-sides, but it would then become necessary to make the end-turn insulation as effective as that on the straight coilsides. This is difficult to accomplish, because the contour of the end-turns is such that it is hard to apply the insulation so that it is free of airvoids. It has become customary, therefore, not to attempt to make the insulating Wrappings or coverings over the end-turns or end-connections as reliable, in dielectric strength, as the insulation of the coil-sides lying within the slots, and hence it has been necessary to utilize insulating materials for the separating blocks or spacers and for the clamping members which are utilized to hold the end-connections in place.

It has been found to be possible to eliminate corona on the end-turns, at the normal voltages which have heretofore been common, by properly spacing the end-connections and by grading off the high-resistance slot-insulation coating and properly distributing the voltage stresses on the portions of the end-turns which are closest to the slots, by means such as are described and claimed in my Patent No. 2,061,502, November 17, 1936.

At higher voltage ratings, however, and particularly at the prescribed test-voltage of twice normal plus 1000 volts, difiiculties have been experienced with end-turn corona on normal machines which have been supplied with ordinary two-layer windings. In a two-layer winding utilizing pre-formed coils, one coil-side of each coil is placed in the bottom of its proper slot, and

this process is completed at least part way around the machine, before the second coil-side of any coil is placed in its proper slot, so that one coilside of each coil lies in the bottom of its slot, and the other coil-side lies in the top of its slot, or in a cylindrical layer which is at a different radial depth than the first coil-side. This type of winding has become the accepted type which ing the coils in the slots, and because of the compactness of its end-connections. It involves difficu1t1es,,however, as a result of corona, in

machines of higher-than-st'andardvoltages, and

particularly at the test-voltages which must be applied thereto before the machines canbe put I :int'o actual use. 7 10' A so-called single-layer winding has also been known, in which all of the coil-sides are at the same radial depth in the various slots, all lying This type of windin a single cylindrical layer. ing has been resorted to, when necessary in, order to reduce the number of coil-sides soas to reduce the voltage of the winding, but it has had the disadvantage of an increased manufacturing cost for labor in assembling the coils, andsomewhat more bulky end-connections in the'fi'nished machine, and greater pole-face losses and other iron losses, particularly with chordings which are materially diiferent from full-pitch coils.

, According to my invention, I utilize a doubledeck single-layer winding,

or two (or more) single-layer windings laidone above the other, and suitably connected in series, so that the coilsides may occupy precisely the same places (except for their order of connection in series) which they would occupy in an ordinary twolayer winding, so that the pole-face losses and other iron losses will be no greater than in a machine having an ordinary two-layer winding. My double-deck single-layer winding, however, produces a machine in which the end-connections of the coils are disposed in four diiferent socalled conical layers, rather than two, so that much greater spacings can'be obtained between adjacentcoil-portions of different voltages, than would be practically possible in a twolayer winding, as will be more fully described hereinafter.

A further important object of my invention is to obtain the benefits of the principle of graded insulation, in a machine utilizing pre-formed coils, or on coils having the same amount of insulation on each of its two coil-sides. The general principles of graded insulation are well known, and call for the gradin'g of the insula-' tion, or the application-ofan' amount of insulation which is commensurate with the maximum voltage appearing on any given portion of the conductor, this grading being done in-two or the neutral point of the generator, thus obtain-' ingthe benefits of:a gradedinsulation in a winding utilizing pre-formed coils; In.this manner,

' that is,;by the useof graded insulation 'in an economical winding which does not require separate bars and end connectors, I can obtain considerable savings in space required for insulation,

and hence in the size and cost of the machine,

7 or I can obtain a much higher voltage-rating for a given size of machine, thus compensating, at least in-pa-rt, for the increased difliculty of winding, and the increased space occupied by the end- 7 connections, which are characteristic of the single-layer winding; The resulting design makes it possible for me to produce a machine of higher-than-usual voltage, with only a relatively slight ing to its class of insulation, that is, twice its normal voltage plus 1000 volts. This involves some slight difficulties, which are in effectno more serious than the necessity for bringing out separate terminal-leads for the two windings, at least temporarily for the period of the test.

The utilization of the principle of graded insulation on' high-voltage polyphase generators also involves greater hazards as a result of excessive voltages such as are produced by lightning or switching operations, particularly light hing. It is an important feature of my present invention that I overcome these hazards by proe viding means, at the neutral point of the genera tor, for providing a ground connection which is effective at least during excess-voltage discharges, and by'providing lightning arresters connected from ground to the several junction points between windings of difierent insulationstrengths, at one or more intermediate points 7 of ,the winding, and'in each of the phases. This is a new principle in lightning-arrester protec tion which constitutes, I believe, a, valuable part of .my solution to the problem of successfully building a generator of higher-than-usual voltage at a minimum cost. The cost of the additional protective equipment is relatively insignificant as compared to the reduction in generator-cost whichxis brought about by the utilization of graded insulation.

Withthe foregoing and other objects in view, my invention consists in the constructions, combinations, connections, systems and methods hereinafter described and claimed and illustrated in the accompanying drawings, wherein Figure 1 is a wiring diagram of one embodiment of my double-deck single-layer winding, with the protective equipment which is utilized for protecting the same against lightning,

Fig. 2 is a somewhat diagrammatic frag mentary longitudinal sectional view of a portion'of the end of a dynamo-electric machine embodying my invention, showing" the bracing of the end-connections, a r

Fig. 3 is' a transversesectional view through the coil-sides lying in one of the stator slots,

Fig. 4 is a diagrammatic view illustrating the circumferential spacing between adjacent endconnections of my single-layer winding,

' Fig. 5 Ba similar View applying to a doublelayer winding, and a Fig. 6 is a view similar to Fig. 1', showing another embodiment. 7

Figs. 1 to 4'illustrate a double-deck singlelayer winding such as is described and illustrated in a paper by C. M. Laffoon and myself a in Electrical, Engineering for June,'l935, vol. 54, pages 624-631. The winding is designed for a three-phase, 'Y-connected, high-voltage, 4- pole generator, comprising a rotor-member 50, a portion of which is indicated in Fig. 2, and a stator-member 5! comprising a magnetizabie slotted core 52 carrying my novel winding, a

development of which is shown inFig. 1. The winding, as shown, consists of two superimposed single-layer windings 53 and 54, of different insulation-strengths, although more than two layers may be utilized, as will be obvious. The coil-sides of these windings, or the portions which lie in the slots 55 of the stator-core 52 (Fig. 3), may be disposed in any order, with the high-insulation winding 54 in either the top or the bottom of the slot: as shown, it is in the top of the slot.

In Fig. l, the low-voltage winding 53 is indicated diagrammatically with dotted lines, and the various low-voltage coil-sides occupying the slots of the stator are indicated by relatively short straight dotted lines 56. The high-voltage winding 54 is illustrated in full lines, and its coil-sides are indicated by relatively long straight full lines 51. The low-voltage and high-voltage windings 53 and 54 are made up of pre-formed coils each consisting of two coilsides 56 and 51, respectively, lying in the slots of the machine, and end-connections 58 at each end of the machine.

The particular machine shown is a four-pole machine having forty-eight slots which are numbered consecutively from 1 to 48 in Fig. 1. Each slot carries a low-voltage coil-side 5S and a high-voltage coil-side 51. For convenience in forming the coils, the coils are shown as being substantially full-pitch coils, having a span of the full-pitch, or 12 slots, plus or minus one slot, which is necessary for the purpose of working out the winding. It is quite feasible, however, to utilize chorded windings rather than substantially full-pitch windings.

In Fig. 1, as is common in winding-diagrams, each coil is illustrated as if it consisted of a sin le turn. As is usual, however, particularly in a. high-voltage machine where the maximum number of turns must be connected in series in order to obtain the high voltage, each coil actually consists of a plurality of turns, and Fig. 1 may be regarded as illustrating, therefore, only the terminal connections of the coils, after the conductor has finished going around, between the two slots in which its coil-sides are disposed, as many times as is necessary in order to complete the requisite number of turns per coil.

In Fig. 3 it is seen that each coil consists of three turns, made up of a conductor 59 which is first lightly insulated, as indicated at 6|, to provide the small amount of insulation which is necessary between turns, the three conductors being subsequently bound together into a single coil, by means of heavier insulation 62, so that each coil is adequately insulated from ground, (assuming the stator-core 52 to be grounded), according to the voltage of the winding of which the coil forms a part. For convenience in fabrication, each conductor 59 is sub-divided into a plurality of strands of flat, uninsulated, copper straps, all lying together and treated as if a single solid conductor had been utilized.

In addition to the coils comprising the coilsides 56 and 51, respectively, and the end-connections 58, previously described, each of the single-layer windings 53 and 54 also comprises a plurality of phase-connecting rings or groupconnectors 63 and 64, respectively.

All of the end-connections, including the end-turns 5B of the coils and the connecting rings 63 and 64, are braced or supported, and insulated from each other, by means of suitable insulating pieces 65 (Fig. 2) mounted on metallic supports 66 which are bolted to the end of the stator-core 52. The end-turns 58 are disposed in a plurality of so-called conical layers as indicated in Fig. 2 by the numerals 87, 68, 69, and 10, the word conical being utilized in its generic sense which includes the limiting case of a plane circle, which is the case in which the end-turns 58 are bent down at right-angles with respect to the coil-sides 55 and 57, as

shown in Fig. 2. It will be understood, or" course, that this angle of bending the end-turns may be less than a right-angle.

In Fig. 1, one complete phase of my doubledeck single-layer winding is indicated in heavy lines, for convenience in tracing out the winding. Starting at terminal-conductor Tl, it will be seen that the heavy-line phase of the winding consists first of two high-voltage coils l2 and 43, res ectively, constituting one pole of that phase of the high-voltage winding 54, the beginnings and ends of these coils being numbered in Fig. 1, The coil '52 has coil-sides lying in slots "i and 25, and the coil 13 has coil-sides lying in slots 8 22. From the coil 73, the circuit continues, through a group-connector T4, to coils l5 and "i8, constituting a second pole of that phase of the high-voltage winding 54, from which the circuit continues, through a group-connector W, to two coils 13 and i9 constituting a third pole of that pha of the high-voltage winding, from which the circuit continues, through a group-connector 8|, to two coils 82 and 83 constituting the fourth pole of that phase of the high-voltage winding, after which circuit is completed, by the endconnector 85, to the junction point J l between the highand low-voltage windings of the phase Tl.

From the junction point J i, the winding circuit is continued through the corresponding phase of the low-voltage winding 53, starting out through the coils 86 and 81 constituting the pole of this phase of the low-voltage winding, and continuing on, through all four poles, to the neutral terminal T4, which is the star point of the Y-connected windings. By a comparison of the low-voltage coils 86 and 8? with the high-voltage coils l2 and 13 of the same pole, it will be observed that the low-voltage coils are displaced two slotpitches to the left of the high-voltage coils, so that it is evident that the low-voltage winding is 2/ 3.2 of or 30 out of phase with the high-voltage winding, thus giving the same efiect as a chorded winding.

Attention has already been called to the fact that the end-turns or end-connections 58 of the coils are disposed in four diiferent axially displaced layers which are indicated in Fig. 2 by the numerals 67 to 10. In each layer of the end-connections the conductors are all inclined the same way around the circumference of the core 1neinher; that is, they do not cross each other. The cross-overs from one layer to another are provided by bends 39 and 93 as indicated in Figs. 1 and 2. In each single-layer winding, in the form shown in Figs. 1 and 2, the end-connections lying in any given layer 6'! to 10 of the end-connections consist of conductors emanating from (or leading to) every other slot, so that the circumferential spacing between these end-connections, in any given layer of the end-connections, corresponds to two slot-pitches of the stator core.

Reference to Fig. 4 will show this spacing of the end-connections more clearly. This figure shows an end View of a few coils of the low-voltage winding 53. The end-portions 92, 93, and 54 lie in the conical end-connection layer 6?; and the end connection layer I58.,, It will be seen that the adjacent end-portions, as 92, $3, in any layer, come from alternate slots 55 of the stator-core 52. a

The significance of the spacing depicted in Fig. 4, for my single-layer winding, will be better appreciated by comparison with Fig. which is a jacent slots 55'.

corresponding view of'a double-layer winding, in

which there are only two conical layers of endconnections at each end of the machine, for the entire winding of the machine. The end-portions IDi, I92, and W3 are in one layer, and the end portions I04, 35, and I538 are in the other layer. It will be noted'that the adjacent end-portions, as IflI and. I92, in any one layer, come from ad- A comparison of Figs. 4 and 5 will show that the center-to-center spacing Ifll between adjacent end-connections in any conical layer or" the'single-layer winding (Fig. 4) is approximately twice the corresponding spacing I28 for the double-layer winding; while the air-spacing Hi9 between the. adjacent single-layer end-connections of the single-layer winding (Fig. 4) 'is ap-' proximately three times. the air-spacing Ill} between the adjacent end-connections in a doublesure,a wide spacing Iiil and IE9 (Fig. 4) between the end-portions of coils of different phases,

which is what is desired, but it also gives the same spacing between the end-portions of coils of the same phase, which is not really necessary,

because the largest voltages appear between coils of different phases rather than between adjacent coils of the same phase. I desire that the embodiment of my invention shown in Fig. 4 shall be taken,'therefore, as an illustration of the principle of securing a wide spacing between conductors of different phases, in any conical layer of the end-connections, regardless of the details or" how the winding is worked out, to bring about this result.

Fig. 6 illustrates the principles just described,

by showing a double-deck single-layer winding in which the adjacent end-portions I I2 and I I3 of coils of the same phase, in any conical layer of the end-connections, are spaced by a single slot-pitch, whereas the adjacent end-portions I93 and H4 of coils of difierent phases have a spacing of three slot-pitches, thereby affording even greater protection against corona than the winding shown in Figszl to i. V c

Fig. 6 also illustrates a different embodiment of my invention in that the individual coils, such as the coil I I2I I5, are chorded to pitch, and the'highand low-voltage windings have their corresponding phases connected together so that the corresponding portions of the two windings are 30 out of phase with each other, therebygiving substantially the same efiect as the windportions 95, 96, 9'], and 98 lie in the conical endthe surges which will surely be present on any line to which the generator is connected. These surges, particularlythose resulting from lightning discharges, which cause the severest surges,

produce two effects on the winding-insulation. In the first place, there is the question of the voltage-stress of-the insulation BI (Fig. 3) between adjacent turnsof a coil, particularly in the coils near the line-ends, during a surge-discharge; and in the second place, there is the question or" a possible breakdown of the insulation between each coil and ground or the frame of the machine.

It has heretofore been demonstrated that the problem of extremely unequal distribution of the surge-voltage between successive coils of the winding, thus tending to produce turn-to-turn breakdown between the over-stressed turns, can

be prevented from being a serious problem if no Waves of steep wave-front are permitted to reach the winding; and hence I utilize a wave-sloper such as is described and illustrated in connection with 'Fig. 14 of a paper; by Fielder and Beck in A. L E. E. Transactions for October, 1930, vol. 49,

pages 1582-1583. This'wave sloper, as shown in 7 Figs. 1 and 6, comprises, for each line-wire LI,

L2 and L3, first a lightning arrester H5, connected from line to ground, for limitingthe magnitride, or the maximum crest-voltage, of the incoming lightning-surge; next an inductance coil tion of the surge-voltage between the successive turns of the winding, thus protecting the turn-toturn insulation.

The problem of protecting the ground-insulation, between each of the coils and ground, against the over-voltages of lightning-surges, is a more serious problem for which I have devised a solution in the form of intermediately connected lightning arresters I2I, I22, and l23 for protectr ing the ground-insulation of the low-insulation part of a graded-insulation winding, as described and illustrated in my article in Electrical Engi-i neering for J anuary,'l934, vol. 53, pages 139-146.

These lightning arresters I2I, I22, and I23 are r connected respectively between the junction points J'I, J 2 and J3 and ground. .They not only limit the maximum possible crest-value of the lightning-surge which may reach the low-voltage winding 53, but they also serve materially to improve the distribution of the surge-voltage be tween the several turns of the various individual coils and also between the several coils of the winding. Thus, if each of the coils of the lowvoltage winding took just a small percentage more than its. proportionate share of the incoming lightning-surge voltage, the voltage-stresses between successive turns of any coil will not be eX- cessive, but the cumulative potential-difference between the last low-voltage coil and ground might well be much more than the low-voltage ground-insulation would stand, in a graded-insulation winding such as mine.

I have found that the intermediately connected arresters'I2I, I22, and I23, when utilized in connection with a suitable neutral-point groundingmeans, such as an impedance device I24 in Fig. 1, or a lightning arrester I25 in Fig. 6, serve to so correct the distribution of the lightning-voltage among the coils of the generator-winding as to prevent these excessive coil-to-ground stresses.

I have not made any effort to illustrate or to describe the corona-preventing coatings which will be utilized both on the straight coil-sides and on portions of the end-connections, as these coatings, while they will preferably be utilized, and probably must be utilized, do not comprise any part of my present contribution to the art.

By a proper application of the principles of my present invention it becomes economically feasible to very materially increase the rated voltages of large alternating-current generators, and to provide a generator-winding which will withstand the test voltage of twice normal plus 1000 volts without any evidences of corona, and which will withstand any surges which may be impressed thereon from lightning-disturbances on the line to which the generator is connected.

I claim as my invention:

1. An alternating-current electric machine having a slotted core-member, and an alternating-current winding having coil-sides lying in the slots of said core member, said winding comprising a plurality of superposed layers of singlelayer windings having a plurality of diverse insulation-strengths.

2. A polyphase electric machine having a slotted core-member, and a polyphase, star-connected winding having coil-sides lying in the slots of said core-member, said winding comprising a plurality of superposed layers of single-layer windings, at least one of said single-layer windings having a lower insulation-strength than another of said single-layer windings and being serially connected therewith, with the low-insulation winding connected nearer to the star point of the polyphase winding.

3. A polyphase electric machine having a slotted core-member, and a polyphase, star-connected winding having coil-sides lying in the slots of said core-member, said winding comprising a plurality of superposed layers of singlelayer windings, each of said single-layer windings comprising formed coils of continuous conductors, at least one of said single-layer windings having a lower insulation-strength than another of said single-layer windings and being serially connected therewith, with the low-insulation winding connected nearer to the star point of the polyphase winding.

4. A polyphase electric machine having a slot ted core-member, and a polyphase, star-connected windin comprising a plurality of superposed single-layer windings; each single-layer winding comprising all of the coil-sides lying in any given cylindrical layer in the slots of the core-member, and two layers of end-connections at each end of the core-member; each layer of end-ccnnections being comprised of coil-portions all inclining the same way around the circumference of the core member; the closest coil-portions of different phases, in each layer of endconnections, being separated circumferentially by a plurality of slot-pitches of the core-member.

5. A pc-lyphase electric machine having a slotted core-member, and a polyphase, star-connected winding comprising a plurality of superposed single-layer windings; each single-layer winding comprising all of the coil-sides lying in any given cylindrical layer in the slots of the core-member, and two layers of end-connections at each endof the core-member; each layer of end-connections being comprised of coil-portions all inclining the same way around the circumference of the core-member; the closest coil-portions of different phases, in each layer of endconnections, being separated circumferentially by a plurality of slot-pitches of the core member; at least one of said single-layer windin s having a lower insulation-strength than another of said single-layer windings and being serially connected therewith, with the low-insulation winding connected nearer to the star point of the polyphase winding.

' 5. A polyphase electric machine having a slotted core-member, and a polyphase, starconnected winding comprising a plurality of superposed single-layer windings each single-layer winding comprising all of the coil-sides lying in any given cylindrical layer in the slots of the core-member, and two layers of end-connections at each end of the core-member; each layer of end-connections being comprised of coil-portions all inclining the same way around the circumference of the core-member, successive coil-portions being separated circumferentially by two slot-pitches of the core member.

7. A polyphase electric machine having a slotted core-member, and a polyphase, star-connected winding comprising a plurality of superposed single-layer windings; each single-layer winding comprising all of the coil-sides lying in any given cylindrical layer in the slots of the core-member, and two layers of end-connections at each end of the core-member; each layer of end-connections being comprised of coil-portions all inclining the same Way around the. circumference of the coremember, successive coil-portions being separated circumferentially by two slot pitches of the coremember; at least one of said single-layer windings having a lower insulation-strength than another of said single-layer windings and being serially connected therewith, with the low-insulation winding connected nearer to the star point of the polyphase winding.

8. A polyphase electric machine having a slotted core-member, and a polyphase, star-connected winding comprising a plurality of superposed single-layer windings; each single-layer winding comprising all of the coil-sides lying in any given cylindrical layer in the slots of the coremember, and two layers of end-connections at each end of the core-member; each layer of endconnections being comprised of coil-portions all inclining the same way around the circumference of the core-member; the coils per phase per pole, in each of the single-layer windings, being disposed in adjacent slots; and the end-portions of adjacent coils of difierent phases, in any layer of end-connections, being separated by more than two slot-pitches.

9. A polyphase electric machine having a slotted core-member, and a polyphase, star-connected winding comprising a plurality of superposed single-layer windings; each single-layer winding comprising all of the coil-sides lying in any given cylindrical layer in the slots of the core-member, and two layers of end-connections at each end of the core-member; each layer of end-connections being comprised of coil-portions all inclining the same way around the circumference of the core-member; the coils per phase per pole, in each of the single-layer windings, being disposed in adjacent slots; the end-portions of adjacent coils of different phases, in any layer of two slot-pitches; at least one of said single-layer windings having a lower insulation-strength than another of said single-layer windings and being serially connected therewith, with the low-insu lation winding connected nearer to the star point of the polyphase winding.

terminal junction-point between two'windings of. diiferent insulation-strengths at some intermediate-point in the machine-winding. V

11. A polyphase electric machine having a slotted core-member, and a polyphase, star-connected winding comprising a plurality of superposed single-layer win-dings, each single-layer winding comprising coils having a pitch which is within one slot of being full-pitch, at least one of said single-layer windings having. a lower insulation-strength than another of said singlelayer windings and being serially connected therewith, with the low-insulation winding connected nearer to the star point of the polyphase winding.

7 12. A polyphase electric machine having :a

slotted core-member, and a polyphase, star-connected winding comprising a plurality of super posed single-layer windings having coil-sides lying in the slots of said core-member, one of said single-layer windings being star-connected, and the phases of the other single-layer windings being serially connected 'to the corresponding phases of said star-connected winding, with a slight phase-difference between at least two of said single-layer windings.

13. A polyphase electric machine having a slotted core-member, and a polyphase, star-conend-connections, being separated by'more than ing being star-connected and the phases of an-.

other single-layer winding being serially connected to the phases of the lowest-insulation winding, with a slight phase-difference between at least two of said single-layer windings.

14. A polyphase electric machine having a slotted core-member, and a polyphase, star-connected winding comprising a plurality of superposed single-layer windings; each single-layer winding comprisingchorded coils of less' than 7 full pitch; and. series-circuit connections em-' bracing first all of the coils corresponding approximately to a particular phase in one layer,

and then all of the coils corresponding approx- 7 imately to said phase in another layer.

15. A polyphase electric machine having a slotted core-member, and a polyphase, star-connected winding comprising a plurality of superposed single-layer windings; each single-layer winding comprising chorded coils of less than full pitch; at least one of said single-layer windings having a lower insulation-strength than another of said single-layer windings and being serially connected therewith, with the low-insulation winding con-nected nearer to the star point of the polyphase winding.

16. A polyphase electric machine having a slotted core-,member, and a polyphase, star-connected winding comprising a, plurality of superposed single-layer windings; each single-layer winding comprising chorded coils of less than full pitch; at least one of said single-layer Windings having a lower insulation-strength than anphase-difierence between the serially connected single-layer windings.

. JOHN F. CALVERT. 

